Field of the Invention
The present invention relates to in-situ fixation of metal contaminants in soil and/or groundwater.
Description of the Related Art
The treatment of contaminated soils and groundwater is an important aspect of environmental science and particularly, is relevant in respect to uncontrolled hazardous waste disposal sites. The most common methods for site remediation include excavation and landfill disposal. While these methods are known to remove contaminants, both methods are costly and in some cases difficult if not impossible to perform. More recently, research has focused on the conversion of contaminants contained in soil and groundwater based upon the development of on-site and in situ treatment technologies. These technologies focus on the removal of contaminants through physical methods or the in-situ treatment of contaminants utilizing chemical or biological methods.
Physical methods of removing metallic contaminants focus on the utilization of pump and treat technology. Chemical methods of removing metallic contaminants focus on chemical changes of valence states, either reduction or oxidation, so as to convert the metallic contaminants to less harmful or less mobile states. The use of strong reductants have been shown to be effective at converting the valence state of chromium VI, a very mobile and toxic form of chromium, to chromium III, a less mobile and less toxic form.
Arsenic as a metal contaminant demonstrates a more mobile valence state, arsenic III (As+3) that migrates freely in groundwater and is particularly toxic. Consequently, in-situ arsenic treatment in soil and groundwater has focused on the binding or the fixing the arsenic to an iron complex that precipitates and either can be filtered from water in an aboveground treatment system, or left to settle within the soil matrix under in-situ conditions. The conventional arsenic treatment process is straight forward within a drinking water treatment facility. Specifically, in a conventional arsenic treatment process, extracted groundwater is mixed with an iron solution and hydrogen peroxide; the peroxide oxidizes As+3 to As+5, the dissolved oxygen concentration increases due to the addition of the peroxide, iron then forms oxyhydroxide using the dissolved oxygen and the As+5 binds to iron-oxyhydroxide. This entire complex is a solid and is then filtered from the groundwater.
The treatment of arsenic by the conventional process is much more difficult outside of a wastewater treatment plant and under in-situ conditions since distribution and mixing of the separate solutions, iron and peroxide, is impractical due to the rapid nature of the reactions and the instability of hydrogen peroxide when in contact with organic material within the soil matrix. Under in-situ conditions, the reactions occur so quickly after injection into the groundwater-saturated soil matrix and the peroxide decomposes to oxygen and water rapidly producing abundant volumes of gas within the soil matrix. It is possible to conduct a treatment program with these methods, however the distribution radius around the injection location is very limited and the injectable volumes are small. These limitations are due to the rapid reaction rates which produce rapid and significant volumes of gas which often cause fluid, groundwater and injected solutions, to surface or daylight soon after injection commences.